[Ag(NH3)2]2SO4: A Strategy for the Coordination of Cationic Moieties to Design Nonlinear Optical Materials

Over recent decades, guided by the anionic group theory of nonlinear optical (NLO) materials, rational design strategies have been primarily focused on anionic moieties; consequently, structural modification and design of cationic moieties have long been neglected. Herein, we report a strategy for the coordination of cationic moieties that substantially enhances the optical properties of NLO materials. For an example with well‐known crystal structure, [Ag(NH3)2]2SO4, we demonstrate that the coordination of the Ag+ cation by the neutral ligand drives the formation of a noncentrosymmetric tetragonal P4‾ 21c structure as a positive uniaxial crystal. The bending of the [Ag(NH3)2]+ cationic moiety parallel to the z‐axis generates an anisotropic arrangement of the dipoles, i.e., a dipole of 0.12 D along the z‐direction, yet zero dipole in the xy‐plane, which interacts anisotropically with the incident light oscillating electromagnetic wave, leading to optical anisotropy with a large birefringence. The incident beam of 589.3 nm normal to the (110) crystal plane measures Δnobv.=0.08, and [Ag(NH3)2]2SO4 also exhibits a phase‐matching NLO response 1.4 times that of KH2PO4 (KDP) (obv. 1.4×KDP @1064 nm; cal. d36=1.50 pm V−1). A new strategy for the coordination of cationic moieties offers a new possibility for the rational design and property modification of functional ionic materials, particularly NLO materials. As demonstrated by [Ag(NH3)2]2SO4 and Ag2SO4, the exceptional enhancements of the linear birefringence and nonlinear SHG properties are attributed to the unique [Ag(NH3)2]+ cation moiety.